1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor for use in image forming apparatus such as copiers, facsimile machines, laser printers, and digital plate making machines. In addition, the present invention also relates to an image forming apparatus and process cartridge using the photoreceptor.
2. Discussion of the Background
Electrophotographic image forming methods using a photoreceptor, which are used for copiers, facsimile machines, laser printers, direct digital plate making machines etc., are well known. The image forming methods typically include the following processes:    (1) charging an electrophotographic photoreceptor (charging process);    (2) irradiating the charged photoreceptor with imagewise light to form an electrostatic latent image thereon (light irradiating process);    (3) developing the latent image with a developer including a toner to form a toner image thereon (developing process);    (4) optionally transferring the toner image on an intermediate transfer medium (first transfer process);    (5) transferring the toner image onto a receiving material such as a receiving paper ((second) transfer process);    (6) fixing the toner image to fix the toner image on the receiving material (fixing process); and    (7) cleaning the surface of the photoreceptor (cleaning process).
Currently, image forming apparatus such as copiers, facsimile machines and laser printers tend to be for private use. Therefore, a need exists for miniaturized image forming apparatus. In addition, image forming apparatus having good reliability, i.e., maintenance-free image forming apparatus are also needed.
In addition, currently image scanners and image processing apparatus such as computers are dramatically improved, and therefore it becomes possible to prepare images having high resolution. Therefore, a need exists for image forming apparatus which can stably produce images having high resolution.
Until now, the following photoreceptors are known:    (1) photoreceptors in which a layer including an inorganic photosensitive material such as selenium or amorphous silicon is formed on an electroconductive substrate as a photosensitive layer;    (2) photoreceptors using an organic photosensitive material;    (3) photoreceptors using a combination of an inorganic photosensitive material and an organic photosensitive material; and    (4) photoreceptors using organic photosensitive materials.
Currently, the photoreceptors using organic photosensitive materials are widely used because of having the following advantages over the other photoreceptors:    (1) manufacturing costs are relatively low;    (2) it is relatively easy to design a photoreceptor having a desired property (i.e., the designing flexibility of a photoreceptor can be increased); and    (3) hardly causing environmental pollution.
As the organic photoreceptors, the following photoreceptors are known:    (1) photoreceptors having a photosensitive layer including a photoconductive resin such as polyvinyl carbaozole (PVK) or the like material;    (2) photoreceptors having a photosensitive layer including a charge transfer complex such as a combination of polyvinyl carbaozole (PVK) and2,4,7-trinitrofluorenone (TNF) or the like material;    (3) photoreceptors having a photosensitive layer in which a pigment, such as phthalocyanine or the like, is dispersed in a binder resin; and    (4) photoreceptors having a functionally-separated photosensitive layer including a charge generation material and a charge transport material.
Among these organic photoreceptors, the photoreceptors having a functionally-separated photosensitive layer especially attract attention now.
The mechanism of forming an electrostatic latent image in the functionally-separated photosensitive layer having a charge generation layer and a charge transport layer formed on the charge generation layer is as follows:    (1) when the photosensitive layer is exposed to light after being charged, the light passes through the transparent charge transport layer and then reaches the charge generation layer;    (2) the charge generation material included in the charge generation layer absorbs the light and generates a charge carrier such as electrons and positive holes;    (3) the charge carrier is injected to the charge transport layer and transported through the charge transport layer due to the electric field formed by the charging;    (4) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image.
For such functionally-separated photoreceptors, a combination of a charge transport material mainly absorbing light having a wavelength in an ultraviolet region and a charge generation material mainly absorbing light having a wavelength in a visible region is effective and is typically used.
However, it is well known that the functionally-separated organic photoreceptors have a drawback of having poor mechanical and chemical durability. This is because low molecular weight charge transport compounds, which have been typically developed and used as the charge transport material, do not have film forming ability. Therefore, a combination of an inactive polymer and a low molecular weight charge transport compound is typically used for the charge transport layer. However, such a charge transport layer is soft, and therefore has also poor mechanical durability. When such a photoreceptor is repeatedly contacted to various elements such as developer, developing roller, transfer paper, cleaning brush and cleaning blade, the surface of the photoreceptor is easily abraded due to the mechanical stress applied by the elements.
In addition, the organic photoreceptors have another drawback such that they easily react with active substances (i.e., corona discharge induced products) such as ozone and nitrogen oxides (NOx), which are generated when charging the photoreceptors in the charging process essential to electrophotography, resulting in deterioration of charge properties of the photoreceptors and occurrence of undesired images such as tailing and blurring. In particular, in order to prepare a photoreceptor which can produce images having good resolution and which have good durability and stability, this drawback has to be remedied.
In attempting to remedy the former drawback (poor mechanical durability) of such an organic photoreceptor, the following techniques have been disclosed:    (1) a brush is used instead of a blade in the cleaning process, in which the photoreceptor is subjected to the largest mechanical stress, to reduce the mechanical stress; and    (2) a lubricant applying device is provided in the vicinity of a photoreceptor, which device applies a lubricant on the surface of the photoreceptor, to decrease the abrasion of the photosensitive layer of the photoreceptor (this technique has been disclosed in Japanese Laid-Open Patent Publications Nos. 6-342236, 8-202226 and 9-81001).
The abrasion can be improved by these techniques to some extent, however, the latter drawback (i.e., poor resistance to ozone and NOx) cannot be remedied. Therefore these techniques are not satisfactory.
In attempting to remedy the latter drawback of the organic photoreceptor, the following techniques have been disclosed:
(1) Contact Charging Methods
The charging methods for charging a photoreceptor are classified into two types, one of which is non-contact charging methods and the other of which is contact charging methods.
Among the non-contact charging methods, a corona discharging method is well known in which a photoreceptor is charged using an electroconductive element, such as wires and plates, which is provided apart from the surface of the photoreceptor and to which a high voltage is applied. This method has an advantage in that the surface of a photoreceptor can be uniformly charged, and therefore the method has been typically used.
On the contrary, in the contact charging methods, a photoreceptor is charged by a charging element, such as brushes, roller-shaped brushes, rollers, blades and belts, which has an appropriate electroconductivity and elasticity and which contacts the surface of the photoreceptor. These methods have been disclosed in Japanese Laid-Open patent Publications Nos. 63-149668 and 7-281503.
The contact charging methods have an advantages over the non-contact charging methods in that the voltage applied to the photoreceptor can be reduced and thereby the amount of generated ozone, which is considered to damage human beings and photoreceptors, can be reduced. Therefore, recently these contact charging methods have widely spread.
(2) Short Range Charging Methods
As intermediate methods between the contact charging methods and non-contact charging methods, short range charging methods in which a DC voltage overlapped with a DC or AC voltage is applied to a photoreceptor using a charging element, such as a brush, a roller-shaped brush, a roller, a blade or a belt, which has an appropriate electroconductivity and elasticity, while a narrow gap is formed between the charging element and the photoreceptor. These short range charging methods are practically used recently.
When an organic photoreceptor is used, it is effective to use the contact charging methods or short range charging methods because of having the following advantages:    (1) having high charge efficiency;    (2) generation of corona-discharge-induced products such as ozone and NOx can be reduced, resulting in prevention of occurrence of undesired images such as blurring and tailing, thereby prolonging the life of the photoreceptor.
With respect to the contact charging methods or short range charging methods, various methods have been disclosed in, for example, Japanese Laid-Open Patent Publications Nos. 56-104351, 57-178267, 58-40566 and 58-150975.
However, generation of the corona-discharge-induced products cannot be perfectly avoided even when these methods are used. Therefore, high durability and stability cannot be imparted to an organic photoreceptor only by using these methods.
In addition, in attempting to impart resistance to the chemical and electrical stresses to an organic photoreceptor, techniques in which an additive is added to the photosensitive layer of the photoreceptor. For example, Japanese Laid-Open Patent Publications Nos. 6-83097, 7-152217 and 7-84394 have disclosed techniques in which a fluorine-containing resin is included in a top layer such as a photosensitive layer or a protective layer to control the surface energy of the layer, resulting in improvement of the chemical durability of the photoreceptor. However, desired durability cannot be imparted to the photoreceptor even when the addition quantity of such an additive is changed. In addition, there is a possibility that such an additive adversely affects the properties of the photoreceptor such as electric property and the like.
Because of these reasons, a need exists for an electrophotographic photoreceptor which can produce images having good image qualities and which has high durability and stability.